1. Field of the Invention
The present invention relates to a silicon single crystal which is produced by crucible-free float zone pulling and has a large diameter.
2. The Prior Art
Pulling silicon single crystals by crucible-free float zone pulling (floating zone crystal growth, FZ pulling) is a technique which has long been known. In this technique, a polycrystalline stock ingot is gradually melted down with the aid of a high-frequency coil, and the molten material is converted into a single crystal by seeding with seed crystal in single crystal form and subsequently recrystallization. The principles of this process are described, for example, in DE-3007377 A.
Despite long years of experience gained using this technique, it has not hitherto been possible to pull dislocation-free single crystals with a diameter of significantly more than 150 mm. This objective is confronted by a whole range of difficulties. For example, relatively high electrical power is required to melt down the stock ingot, and consequently there is a particularly high likelihood of electrical spark-overs in the region of the coil supply conductors. Spark-overs of this type may end the single-crystal growth and are therefore to be avoided. Another difficulty is that of achieving a dimensionally stable growth of the single crystal which leads to the appearance of the single crystal being as far as possible cylindrical. This is because it has been found that, when it is attempted to pull single crystals with a large diameter, the growth front can all too easily break out in the radial direction. This leads to misshapen single crystals which cannot be processed into wafers or can only be processed into wafers with difficulty. However, the main problem is the considerable likelihood of dislocations forming.